Because lettuce has a short shelf life and the last illness reported started more than a month ago, the CDC said in its final update it is likely the contaminated leafy greens liked to the outbreak are no longer for sale.

The FDA said it will continue “to work with federal, state and local partners to determine what leafy greens made people ill, what people ate, where they bought it and identify the distribution chain,” with the goal of identifying where the food might have become contaminated.

“It is very difficult to remove bacteria from leafy greens,” James Rogers, Ph.D., director of food safety and research at Consumer Reports told the magazine. “Bacteria have the ability to adhere to the surface of the leaves, and to get stuck in microscopic crevices.”

E. coli bacteria specifically has unique survival tactics that make it impossible to wash away. It produces a protective biofilm that encases the bacteria and helps it adhere to a surface. It can also penetrate into produce, burrowing below the surface.